1. Field of Invention
The invention relates to a variable gain amplifier with a gain adjusting circuit, and, in particular, to a variable gain amplifier, which has a gain adjusting circuit and is applied to a complementary metal-oxide semiconductor.
2. Related Art
With the advancement in communication technology, wireless communication has been widely used in various electronic products. Now, communication systems have entered a third generation of mobile communications (Wideband Code Division Multiple Access, hereinafter referred to as WCDMA), such that the demands on communication quality have further increased. Furthermore, the internal circuits in the WCDMA communication systems need amplifiers, and the quality of the amplifier directly influences the communication quality.
A variable gain amplifier (VGA) will be hereinafter referred to as an amplifier. Gain is an important factor of an amplifier. Referring to FIG. 1, a conventional amplifier 1 includes an exponential current transforming circuit 11, a voltage buffer circuit 12 and an amplifying circuit 13. The voltage buffer circuit 12 is coupled with the exponential current transforming circuit 11 and the amplifying circuit 13. In addition, the amplifier 1 operates as follows. The exponential current transforming circuit 11 receives a current control signal Icl from an outside source, transforms the current control signal Icl into a current output signal Io and outputs the current output signal Io to the voltage buffer circuit 12. The voltage buffer circuit 12 outputs a voltage controlling signal Vcl to the amplifying circuit 13 according to the current output signal Io. After the amplifying circuit 13 receives an input signal INPUT from the outside, the amplifying circuit 13 applies the desired gain to the input signal INPUT, thus amplifying the input signal INPUT according to the adjustment of the voltage controlling signal Vcl and then outputting a signal OUTPUT.
Usually, the amplifying circuit 13 of the amplifier 1 is implemented by BJT transistors. However, the manufacturing processes of BJT transistors are more expensive, and are not easily compatible with the complementary metal-oxide semiconductor (CMOS) processes. Therefore, BJT transistors are gradually being replaced with MOS transistors. Referring to FIG. 2, an amplifier 2 implemented using the MOS transistors includes an exponential current transforming circuit 21, a voltage buffer circuit 22 and an amplifying circuit 23, and the operations thereof are the same as those of the amplifier 1 in FIG. 1. When the MOS transistors of the exponential current transforming circuit 21 work in sub-threshold regions, the same effects as those in the amplifying circuit 13 using the BJT transistors may be achieved. At this time, the transistors Q1 and Q2 of the voltage buffer circuit 22 work in saturation regions, and the current gain G is:
  G  =                    {                  1                      1            +                          2              ⁢                                                                    I                    ⁢                                                                                  ⁢                    1                    ⁢                    I                    ⁢                                                                                  ⁢                    2                                                  IB                                                    }                    1        2              *                  exp        ⁡                  (                                                    -                R                            ⁢                                                          ⁢              1              *              Icl                                      2              ⁢                              nV                T                                              )                    .      If the transistor Q1 works in the sub-threshold region and the transistor Q2 works in the saturation region, the current gain G becomes:
  G  =            {                        IB                          2          ⁢                      nV            T                    ⁢                      β                              }        *                  exp        ⁡                  (                                                    -                R                            ⁢                                                          ⁢              1              *              Icl                                      nV              T                                )                    .      Wherein, G is the current gain, IB is a bias current, I1 and I2 are the currents respectively flowing through the transistors Q1 and Q2, R1 is the resistance, Icl is the current control signal, and VT is the threshold voltage.
Due to the demand on the specification of the WCDMA communication system, the current gain G needs to be exponentially linear when amplifiers composed of MOS transistors is used. However, the obtained current gain G is exponentially non-linear in practical application no matter which regions the transistors Q1 and Q2 are operating in. For example, FIG. 3 shows the relationship between the current gain G and the current control signal Icl when the amplifier composed of the MOS transistors is being used, wherein the curve with the slope A represents the ideal current gain G when the transistors Q1 and Q2 work in the saturation region, and the curve with the slope B represents the ideal current gain G when the transistor Q1 works in the sub-threshold region and the transistor Q2 works in the saturation region. In practice, however, when the current gain G is high, the current gain G differs from the curve with the slope A by about 3 dB, as shown by the solid line. In order to compensate for the difference of 3 dB, a gain compensation circuit is usually provided to compensate for the current gain. The conventional gain compensation circuit is large and complex, greatly complicating system design.
Therefore, it is an important subject of the invention to provide a variable gain amplifier, which has a gain adjusting circuit and can solve the above-mentioned problems.